Burglar alarm systems generally fall into one of two types Perimeter Systems that protect the perimeter with sensors on doors, windows, and other openings, and Space Systems that protect an area or space by use of motion detections, i.e. ultrasonic or microwave, light beams, trip wires, etc. Space Systems, particularly for home use, are generally of the motion detection type. This type of burglar alarm detects movement in an area rather than detecting entry into the area through an opening. Because the volume of the space that can be monitored by a single unit is limited, a multiplicity of units is generally required, which causes this type of system to be costly. Further, this type of system, because it detects all motion, tends to have a high false alarm rate.
The most commonly used perimeter system is based on the use of switch devices at the entry points. These switch devices are wired together in series and the circuit is terminated at a central monitor, generally some form of bridge circuit coupled with a latching circuit. If the series circuit is broken, the central monitor detects the open circuit condition and causes the latching circuit to latch and to activate an alerting or signaling device. This system is simple in theory, reliable in practice, and has a low false alarm rate if properly used. However, because of the extensive wiring needed and the normal desire to conceal the wiring (particularly in homes), this type of system is difficult and expensive to install.
Another commonly encounted perimeter system that removes the need for connecting wires, uses sensors at each opening that are based on switches to detect the unauthorized opening, and a low power radio frequency transmitter to send a signal to a central receiver if entry occurs. This system circumvents the need to install extensive wiring and is, therefore, less expensive than the wired perimeter system. However, each sensor is a complex electronic unit, and such units are expensive. Also, because each sensor must contain a battery to provide power, the sensors are sensitive to power drain and tend to be unreliable. These systems unless they use complex, and therefore more costly, coding schemes tend to have a high false alarm rate.
From the above discussion, it can be seen that a type of alarm system that is inexpensive, easy to install, reliable, and possessed of a low false alarm rate is needed. It is the purpose of this invention to provide a burglar alarm system having these desirable features.